In this section of the proposed program project grant, we test the hypothesis that the influenza virus NS1 protein may species-specifically suppress the antiviral response of the host cell. Using a recently developed assay which measures interferon-antagonist function of individual proteins, we will compare NS1 function in cells from different hosts, including primary human respiratory epithelial cells and mouse embryo fibroblasts. We will extend these analyses by generating a series of isogenic recombinant influenza viruses encoding mutant NS1 proteins or encoding NS1 proteins from viruses adapted to different species. These viruses will be characterized and tested for growth on different substrates, including transgenic mice with specific defects in their type I interferon response. In collaboration with our colleagues from the University of Washington, the global gene expression profile from these different assay models will be determined. We believe that this approach will answer the question whether the ability of specific NS1 proteins to inhibit the cellular antiviral response influences influenza virus host range and virulence. Also, this approach may elucidate common mechanisms by which respiratory RNA viruses modulate the host's innate immune response. Furthermore, analysis of host-specific NS1 function may allow us to determine which host cell factors must be inhibited or whose expression must be prevented/altered fir efficient viral replication to occur. This should suggest novel antiviral targets and may guide us in developing novel vaccine strategies.